The present invention relates to a circulating fluidized bed combustion system and a heat exchange chamber utilized therein, and, more particularly, to a system in which the heat exchange chamber is provided between a separating section and a furnace section of the circulating fluidized bed combustion system.
Fluidized bed combustion systems are well known and include a furnace section in which air is passed through a bed of particulate material to fluidize the bed and to promote combustion of fuel in the bed at a relatively low temperature. The bed may include fossil fuel, such as coal, sand and a sorbent for the sulfur oxides generated as a result of the combustion of the coal. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relation with the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulfur adsorption and low nitrogen emissions.
In circulating fluidized bed systems, the fluidizing air velocity is such that the gases passing through the bed entrain a substantial amount of the fine particulate solids. External solids recycling is achieved by disposing a particle separator, usually a cyclone separator, at the furnace outlet to receive the flue gases, and the solids entrained therewith, from the fluidized bed. The solids are separated from the flue gases and the flue gases are passed to a heat recovery section while the solids are recycled back to the furnace. This recycling extends the fuel retention and improves the efficiency of utilization of a sulfur adsorbent, thus reducing consumption of both the adsorbent and fuel.
Circulating fluidized beds are characterized by relatively intensive internal and external solids recycling, which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and stabilizing sulfur emissions at a low level. When fluidized bed systems are used to generate steam, the heat released in the exothermal reactions taking place in the furnace may be recovered by heat exchange surfaces disposed in several locations in the system. The walls of the furnace section are usually so-called tube walls, made by welding tubes together with fins. A heat transferring fluid, usually water or steam, is led through the tube walls in order to cool the furnace walls, and to transfer heat therefrom. Other heat exchange surfaces may be located within the system, such as in the walls of a cooled cyclone, in the heat recovery section downstream of the cyclone or in a separate heat exchange chamber, which may be in flow connection with the internal or external recycling of the solids.
The furnace section and the cyclone separator may be bottom-supported, the structure being rigidly supported at its bottom, and the main thermal expansion taking place upwards from the bottom. When designing a large bottom-supported unit, the mechanical loads on the tube walls have to be well considered as the whole weight of the furnace section is transferred through the walls to the lower parts of the boiler, with the tube walls in compressive stress. A significant share of the load may need to be carried from the top steel structure via constant load springs, which may increase the costs significantly.
Therefore, it is, especially in large units, conventional to construct a top-supported furnace and cyclone, i.e., to support them on a steel structure constructed on and above the system, with the main thermal expansion taking place downwards. A top-supported unit is generally easier to assemble than a bottom-supported unit. In top-supported systems, the furnace walls do not have to be stiffened due to the weight of the boiler, because the tube walls can easily endure the tensile stress caused by the load.
The most typical way of manufacturing a heat exchange chamber is to make it of steel plates, which are thermally isolated and protected against wear by a relatively thick layer of refractory material. Such enclosures are cost-effective to construct but, due to different thermal expansions, difficult to join to other units of the system constructed of tube walls. To solve this problem, one has to use flexible joints, such as metal or fabric baffles to accommodate the relative motions between the different parts of the system. Such baffles, however, are expensive and prone to wear.
It is a common practice to construct an external heat exchange chamber as a bottom-supported structure. If the furnace section and the cyclone separator of the system are bottom-supported as well, the relative motions between the different units may be relatively small and the joints therebetween do not have to accommodate large motions. As the heat exchange chamber is typically located near the ground, it is also common, in larger units, to construct the heat exchange chamber as being bottom-supported, while the furnace section and the cyclone separator are top-supported. In such a construction, the relative thermal motions may be very large, and special expansion joints are required to accommodate the motions between the cyclone and the heat exchange chamber and between the heat exchange chamber and the furnace. Typically, these expansion joints are very expensive metal joints.
Another method of constructing a heat exchange chamber is to make its enclosure as a cooled tube wall structure. U.S. Pat. No. 5,911,201 describes a suspending unit comprising a cooled heat exchange chamber integrated with a cyclone separator. U.S. Pat. No. 5,425,412 discloses a method of making a furnace, a cyclone and a heat exchange chamber of tube walls and to integrate them all closely together. In such a system, the temperatures of these units are very close to each other, and thus, due to similar materials and constructions, their thermal expansions are very much alike, and no flexible joints are needed between the units. A drawback in such cooled heat exchange chambers, however, is that the construction, especially if it includes complicated structures and cooled inlet and outlet connections, requires a lot of manual bending and welding of the tubes, and is thus time-consuming and expensive to manufacture. Also, in some applications, the heat exchange chambers tightly integrated with the furnace may take too much space around the lower part of the furnace. This is especially the case in large units, where very high total heat exchange capacity, and, e.g., many fuel feeding ducts, as well, are required in the lower part of the furnace.